Your wireless router broadcasts your Wi-Fi signal on channels ranging from 1 to 13.* As more routers broadcast on the same channel as yours, your router's performance generally decreases. The solution: scan for the least crowded channels and use one of those.
If you're used to seeing countless Wi-Fi networks when you scan for a connection at your home or workplace, there's a good chance your wireless connection is broadcasting on the same channel as those other routers (unless you changed the default). At his home away from Lifehacker, the How-To Geek details how to change your Wi-Fi router channel to optimise your signal using previously mentioned inSSIDer to determine which channels are most and least crowded. He then explains how to switch your broadcasting channel on Linksys routers. (The method will vary from router to router, but finding and changing your router's channel is normally a pretty simple process.)
For an even quicker, download-free method for finding an interference-free channel, check out previously mentioned, web-based WiFi Stumbler. If you've installed the open source Tomato firmware on your router, it scans and lists crowded channels for you, as well. Remember that some Wi-Fi channels do overlap, so keep that in mind when you're choosing the optimum channel for your router. (inSSIDer actually displays some of that overlap.) And while you're tweaking, take a look back at these other settings you should change on your router.
* The number of channels varies by location. Per Wikipedia, the US allows for 11 channels; most of Europe allows 13; Japan goes to 14.